Electric motor wheel brake for vehicle

ABSTRACT

A wheel brake operated by electric motor for motor vehicles, where the operative braking forces and braking torques as well as the actuator control signals are determined for each wheel brake by appropriate measuring equipment. On the basis of these signals, implausible deviations are detected by comparison, and the faulty signal is recognized and treated appropriately by using the corresponding signals of the other wheel brake of the same and/or the additional axle(s).

FIELD OF THE INVENTION

The present invention relates to an electric motor wheel brake for motorvehicles.

BACKGROUND INFORMATION

Electric motor wheel brakes for motor vehicles are known. For example,World Patent No. WO-A 94/24453 describes a wheel brake where the brakeapplication force is generated by an electric motor. If the brake systemof a motor vehicle is composed of such electric motor wheel brakes,special attention must be devoted to reliable functioning of such abrake system to guarantee vehicular driving stability of the vehiclewhen braking in the partial braking range.

Therefore, the object of the present invention is to design a brakesystem of electric motor wheel brakes in such a way as to guaranteedriving stability of the vehicle in braking and uniform brake wear.

SUMMARY OF THE INVENTION

A brake system with electric motor application of brake force is createdto guarantee driving stability of the vehicle even in the partialbraking range.

It is especially advantageous that this permits reliable andfault-tolerant determination of the braking torque and the brakingforces on the wheels. This in turn leads to uniform brake wear in anadvantageous manner.

Special advantages are achieved when using stepping motors orelectronically commutated motors, where wear and tolerance effects canbe compensated by adapting the point of brake application and brakerelease to gradual changes.

It is especially advantageous that a method of reliable monitoring ofsensors for detecting the braking forces is provided with which adefective element can be detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a brake system according to anembodiment of the present invention with electric motor application ofthe brakes.

FIG. 2 illustrates a flow chart of an embodiment, according to thepresent invention, for determining the braking torque values formed ateach wheel brake.

FIG. 3 illustrates a flow chart of an embodiment, according to thepresent invention, for comparing the braking torque values for wheelbrakes on the same axle.

FIG. 4 illustrates a method, according to an embodiment of the presentinvention, for adapting the point of application and release of thebrakes to gradual changes when using stepping motors or electronicallycommutated motors.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a brake system of a motor vehicle withelectric motor application of brake force on the example of an axle.This shows an electronic controller 10 that drives electric motors 16and 18 over output lines 12 and 14. The electric motors are part ofbraking actuators 20 and 22 that act on braking elements 28 and 30 ofwheels 32 and 34 respectively by way of mechanical connections 24 and26. A similar arrangement is provided on additional axles of thevehicle. In the preferred embodiment, electric motors 16 and 18 are d.c.motors. In this case, as indicated with dotted lines, a quantityrepresenting the current flowing through the motors is sent to controlunit 10 over lines 36 and 38 respectively. This quantity is determinedin a known way, e.g., by means of a resistor connected to ground in theH bridge output stage for the d.c. motor. Furthermore, there are forcesensors 40 and 42 whose signals are sent to control unit 10 over lines44 and 46. These force sensors determine the supporting forces of thebraking actuators and in this way determine a measure of the operativebraking forces and braking torques. In the preferred embodiment, thesesensors are wire strain gauges. In other advantageous embodiments, thepressing force of the brake linings is detected by sensors (e.g.,piezoelectric sensors), or the movement of the brake linings or anactuating lever of the wheel brake is determined (e.g., by displacementsensors) as an indirect measure of the braking force or the brakingtorques.

For the sake of thoroughness, FIG. 1 shows input lines 48 through 50connecting control unit 10 to measuring devices 52 through 54. Thelatter detect additional operating quantities of the vehicle or thebrake system, such as wheel speeds, the rotational speed of the driveunit, etc. which are necessary for controlling the brake system.Furthermore, an input line 56 is provided that connects control unit 10to a measuring device 58 for detecting the driver's intent, inparticular for detecting the position of a driver-operated brake pedal.

In an advantageous embodiment, stepping motors or electronicallycommutated motors are used instead of commutator d.c. motors. With suchtypes of motors, detection and supply of the motor current are notperformed because a corresponding quantity is available on the basis ofthe number of steps carried out.

Control unit 10 detects the driver's intent over line 56 and converts itto setpoints for the individual wheel brakes on the basis of apreprogrammed operating map for each wheel brake or groups of wheelbrakes. These setpoints correspond, for example, to braking torques orbraking forces which are to be set as part of a corresponding controlcircuit by driving the electric motors of the wheel brakes. In anadvantageous embodiment, the driver's intent is correlated with thesetpoints as a function of parameters such as axles loads, brake liningwear, brake temperature, tire pressure, etc., whose values are sent tocontrol unit 10 over lines 48 through 50. Furthermore, in specialbraking states, control unit 10 performs the essentially known anti-lockfunctions or traction control functions on the basis of the wheel speedssupplied.

With such brake systems, where the wheel brakes are operatedindividually with an external force, it is important that the brakingforces of the vehicle wheels are set with regard to driving stability sothat the vehicle does not pull to one side and the brakes wear asuniformly as possible. It is therefore necessary for the braking forcesor braking torques acting on the wheels to be detected. Since detectionof these braking forces or braking torques has a direct influence on thebraking action of the vehicle, it is important for the braking torquesand braking forces to be determined reliably and in a fault-tolerantmanner.

The basic idea of the procedure according to the present invention isfor the electronic control unit for controlling the wheel brakes toreceive information with regard to the operative braking torque or theoperative braking force and information regarding the signals sent tothe actuator. The latter information can be used to derive informationregarding the operative braking torque or the operative braking force,so that two items of information are available for the braking torqueand the braking force for each wheel brake.

A preferred embodiment is illustrated on the basis of the flow charts inFIGS. 2 and 3. The basic idea of this procedure is that control unit 10receives from each wheel brake a quantity determined by the force ormotion sensor and receives a quantity representing the motor current or,in the case of stepping motors or electrically commutated motors, aquantity representing the number of steps. In an advantageous manner,the signal of a force or motion sensor is combined with the drivingtorque of the motor actuator for a wheel brake determined on the basisof the actuator-specific quantity. The two signal quantities are checkedfor implausible deviations. In addition, control unit 10 has access tothe corresponding signal quantities of the wheels of the same axleand/or the other axle(s) in normal braking, apart from ABS braking orother special functions.

In other words, control unit 10 is enabled to determine unacceptabledeviations from at least four signal quantities representing the same ora similar braking torque or the same or similar braking force (becausein general the rear axle braking force is lower than the front axlebraking force) and to make corrections when such unacceptable deviationsoccur or to determine which element is defective. This information isthen converted to control commands for the wheel brakes (when there aredeviations between the wheel brakes) and optionally a warning signal isdelivered (when there are deviations within one wheel brake) or a wheelbrake is influenced or even shut down if necessary.

The flow charts diagramed in FIGS. 2 and 3 give suggestions forimplementing this basic idea in a computer program. The procedureaccording to FIG. 2 may also be used in an embodiment alone without theprocedure diagramed in FIG. 3.

The subprogram illustrated in FIG. 2 is initiated at certain timesduring a braking operation. A braking operation is detected, forexample, when a brake pedal switch closes. After the start of thesubprogram, which is carried out for each wheel brake in succession, thesignal values of the force sensor FBrems and the actuator-specificsignal quantity I are entered in a first step 100. In the preferredembodiment, the braking force quantity is a measure of the supportingforce of the brake actuator, while the actuator-specific quantity is thecurrent flowing through the electric motor or the number of stepsdetermined. In the subsequent step 102, the signal values supplied areconverted into braking or driving torque values on the basis ofcharacteristic curves. The supporting force detected by the force sensorcorresponds to a braking torque value M1, taking into account the brakedesign, while the driving torque of the motor M2 is determined on thebasis of the current or the number of steps using predeterminedoperating maps. In the next query step 104, the absolute value of thedifference between these two torque values is compared with a presettolerance value Δ. This value takes into account the tolerance inconversion of the measured values to torque values. If the value of thedifference in torque values is less than the tolerance value, a"deviation" mark is set at the value 0 in step 106, while in the case ofimplausible deviations, when the value of the difference is greater thanthe tolerance value, the mark according to step 108 is placed at thevalue 1. Then the subprogram is terminated and optionally the subprogramshown in FIG. 3 is carried out.

The subprograms shown in FIGS. 2 and 3 may also be carried out with ABSand other special functions in addition to normal braking.

In the preferred embodiment, the subprogram shown in FIG. 3 is initiatedfollowing the subprograms according to FIG. 2. The torque values formedfor each wheel brake are available. According to FIG. 3, differentsubprograms are initiated for the right and left wheel brakes of anaxle, depending on the combination of values of the marks. For example,if an implausible deviation has been detected (MarkeL to 1), while noimplausible deviation is detected for the right wheel of the same axle(MarkeR zero) (150), then the absolute value of the difference betweenthe braking torque M1 on the right wheel and the driving torque M2 onthe left wheel is formed in step 204 in the preferred embodiment and iscompared with a preset tolerance value Δ. As explained above, this valuetakes into account tolerances in the area of the wheel brakes. If theabsolute value of the difference is smaller than the tolerance value, itis assumed according to step 206 that the braking force detection on theleft wheel is functioning properly, whereas according to step 208, thedriving torque M2 determination on the left wheel might be incorrect. Ina preferred embodiment, the torque value derived from the motor currentis regarded as faulty according to step 214, the left wheel brake iscontrolled exclusively on the basis of the braking force signal and thissignal is monitored according to step 204 by comparison with the torquevalue derived from the motor current or from the braking force value ofthe right wheel brake.

Furthermore, the driver is notified of the inconsistency by a warningsignal.

However, an advantageous supplementary measure yields the possibility ofverifying the result of steps 206 and 208, because four quantitiesindicating the same torque are obtained on one axle. Therefore, in anadvantageous embodiment, the torque value derived from the motor currentor the number of motor steps for the right wheel is compared with thetorque value derived from the braking force detected on the left wheel.If the result of steps 206 and 208 is confirmed in query step 212, themeasures described above are initiated according to step 214. If theverification does not confirm the result of steps 206 and 208, theresult is first ignored according to step 216 and an error message isgenerated when several inconsistencies occur in succession in anadvantageous embodiment. Then the subprogram is terminated and isrepeated in conjunction with FIG. 2.

If it was determined in query step 204 that the absolute value of thedifference between the two values is greater than the preset tolerancerange, then it is assumed according to step 218 that the torque valuethat has been derived from the motor current is correct, and accordingto step 220, the torque value derived from the braking force detected isincorrect. In the preferred embodiment, this leads according to step 222to controlling the left wheel brake as a function of the torque valuederived from the motor current. This torque value is monitored on thebasis of the corresponding values for the right wheel, where if thereare inconsistencies, the faulty signal can again be determined andisolated in this case on the basis of a two-of-three selection.Furthermore, here again a warning signal is generated. According tosteps 210, 212 and 216, it is advantageous to verify the results on thebasis of a comparison of the torque value derived from the motor currentfor the right wheel with the torque value derived from the braking forcefor the left wheel according to steps 224, 226 and 228.

If the mark of the left wheel brake is zero and that of the right wheelbrake is 1 (151), the steps corresponding to steps 204 to 228 arecarried out according to step 154.

If both marks have a value of zero (156), a correct determination of thebraking torques on the wheel brakes of an axle can be assumed. Thereremains, however, some uncertainty with regard to deviations between thewheel brakes that can be attributed to different brake lining wear, tirepressures and tire wear, aging phenomena, etc. therefore, in query step158 in the preferred embodiment, the brake torque on the left wheel iscompared with that on the right. If the absolute value of the differenceis within a tolerance range, this ensures that the torque detection iscorrect. If the absolute value of the difference is outside thetolerance range, the control signal preferably for the wheel brake withthe lower torque value is corrected according to step 160 to make thetorque values on the right and left wheel brakes coincide. In thepreferred embodiment, this is done by correcting the setpoint; in otherembodiments this can also be done by correction of the measurementsignal (actual value). The subprogram is terminated after step 158 or160.

If both marks have a value of 1 (157), i.e., if implausible deviationshave occurred on both wheel brakes of one axle, the faulty signals aredetermined according to step 162 as in the procedure according to FIG. 2and steps 150 through 160 and 204 through 228 by comparison with thesignals of another axle (optionally with wider tolerances), preferablythe signals of the wheel brakes of different sides of the vehicle.

A subprogram corresponding to the subprogram according to FIG. 3 is runfor the other axles of the vehicle.

With the procedure described here, information about the braking torqueand braking force from measured signals and sent signals (actuator) iscompared with minimal extra expense, so that in the case of partialbraking, four signal values from two wheels on the same axle arecompared for implausible deviations. If the signal reliability is to befurther reinforced, similar signal values are also available from thewheel brakes of the other axles of the vehicle. Signal reliability cantherefore be additionally reinforced by comparison with these signalvalues, optionally with a wider tolerance range.

In addition to the method presented here, a comparison of the torquevalues derived from the braking force values or the torque valuesderived from the motor currents is also advantageous in steps 204, 158,etc. in other advantageous embodiments.

In addition to the use of the motor current to determine the drivingtorque, the number of steps executed SZ is used as a measure of thedriving force and thus for the braking force in an advantageousembodiment when using stepping motors or electrically commutated motors.

In contrast with the use of the motor current, however, the followingspecial case occurs here. When using such brake application devices, thepoint of brake application and release shifts with respect to the numberof steps because of wear in particular. Therefore, this point ofapplication or release must be determined continuously in conjunctionwith the determination of braking force and braking torque. This is donethrough the subprogram outlined in FIG. 4. The determination of theapplication point can also be used with commutator motors.

When initiating a braking operation, the subprogram shown in FIG. 4 isrun through at least once per operating cycle. The brake control isactivated by operating the brake pedal and a braking actuation as perthe driver's input is initiated by control unit 10. According to FIG. 4,to determine the application point, the applied braking force and thecontrol quantity are entered while this braking control operation istaking place. In the next step 302, the application point of the wheelbrake is determined on the basis of the relationship between the brakingforce and/or the control quantity. For this purpose, the braking forcevalue thus entered is compared with a preset limit characterizingactuation of the brake. In other advantageous embodiments, a break inthe braking force curve is used to detect the application point, or theapplication point is extrapolated (braking force 0) on the basis of therelationship between braking force and control quantity (characteristiccurve). If the application point is not detected in step 302, a check isperformed in step 304 to determine whether the subprogram is to beterminated. For example, this is the case when the application point isnot detected during a predetermined period of time or the brakingoperation has been terminated. If the termination condition does notexist, the subprogram is repeated with step 300.

If the application point is detected in step 302, then in step 306, theprevailing step count SZ or current value at the moment is detected, andin step 308 the application point of the brake SZA is stored on thebasis of the step count SZ entered and optionally a tolerance value Δ.Then in step 310, the zero point of the step counter when the brake isreleased and the brake pedal is released (driver's intent zero) SZ0 isentered, and in step 312 and in the control program, the braking forceis derived from the instantaneous step count SZ, from which theapplication point SZA and zero point SZ0 are subtracted.

A similar procedure is followed to determine the release point.

What is claimed is:
 1. A braking element with an electric motor brake ofa motor vehicle having at least two wheels, comprising:an electric motoractuating element for each of at least two wheel brakes the electricmotor actuating element including at least one electric motor and anactuator; a controlling system coupled to the electric motor actuatingelement, the controlling system activating the electric motor actuatingelement via triggering signals, the controlling system receiving from ameasuring device first quantities corresponding to one of brakingtorques and braking forces acting on the at least two wheels, andreceiving from the electric motor actuating element second quantitiesinfluenced by the controlling system, the second quantities representingthe one of braking torques and braking forces acting on the at least twowheels, the controlling system including an arrangement detectingunacceptable deviations between the one of braking torques and brakingforces by comparing the first and second quantities, and determining anerror status as a function of the unacceptable deviations.
 2. Thebraking element according to claim 1, wherein the measurement deviceincludes at least one of i) measurement equipment which detects asupporting force of the actuating element, ii) measurement equipmentwhich detects a pressing force of brake linings, and iii) measurementequipment which detects a movement of one of the brake linings and anactuating lever of the wheel brakes.
 3. The braking element according toclaim 1, wherein the second quantities include at least one of atriggering signal quantity supplied to the actuating element, a currentflowing through the at least one electric motor of the actuatingelement, and a number of steps of the at least one electric motor. 4.The braking element according to claim 1, wherein, when the unacceptabledeviations are detected, a comparison is performed on a basis of twoquantities with corresponding quantities of a wheel brake on a same axleto determine a faulty quantity.
 5. The braking element according toclaim 1, wherein unacceptable deviations are detected by comparisonsbased on the first quantities and the second quantities of the wheelbrakes of different axles.
 6. The braking element according to claim 5,wherein a test for the unacceptable deviations in a normal braking isperformed without an intervention of a stop control system, otheradditional information, an ABS control, and other special functions. 7.The braking element according to claim 1, wherein at least one of aprevailing point of application and a point of release is determined bythe controlling system from a relationship between one of the secondquantities and an operative braking force.
 8. The braking elementaccording to claim 1, wherein, when an error is detected, a faultyquantity is determined and a brake system is switched off with regard toa remaining control.
 9. The braking element according to claim 8,wherein, when the error is detected, the faulty quantity is determinedand a comparison for the unacceptable deviations with the faultyquantity at one of the wheel brakes is derived from a correspondingquantity of another of the wheel brakes of a same axle and a remainingcorrect quantity of the one of the wheel brakes.